Conventional flow cytometers have been widely used in the analysis of blood cells. Such a blood cell cytometer is mainly comprised of an illuminating unit, a flow chamber and a signal processing unit. The flow chamber provides an optical cell-interrogation zone, in which a flow of the sample of blood cells is encircled in a sheath flow according to the fluid focusing principle, so that the blood cells pass through the detection passage one by one; the illuminating unit, usually a laser, provides an illuminating light beam which may irradiate into the cell-interrogation zone of the flow chamber, such that the illuminating light beam may irradiate onto the cells flowing through the cell-interrogation zone so as to be scattered, or excite fluorescence emission, etc.; and the signal processing unit is useful for collecting various optical information generated in the flow chamber and converting it into electric signals. By processing and analyzing these converted electric signals, the parameters of various cells contained in the blood can be obtained in order for subsequent processing such as counting and classification, etc.
Normally, certain properties of cells are all represented by the peak or pulse width of the signals described above, thus it is necessary to obtain such data as the peak or pulse width of various optical information, and some parameters concerning the blood cells may be calculated by using a histogram or scatter diagram plotted with these data.
In prior art, as shown in FIG. 1, a typical flow cytometer has to pass the light beam LB eradiated from a light source through the optical system which focuses the beam on the center of the cell-interrogation zone of the flow chamber 4, forming an elliptical spot BS on a plane π perpendicular to the optical axis at the center of the cell-interrogation zone, and the minor axis of the spot should coincide with the flowing direction {circumflex over (ƒ)} of the sample. Since the light beam has a very large divergence angle θy in the direction of the cell flow, a significant aberration in particular a spherical aberration occurs after the light beam passes through the optical system, such that the spot focused at the cell-interrogation zone of the flow chamber, besides a main spot BS0 in the sample flowing direction, also has two symmetrical sidelobes BS1 and BS2, as shown in FIG. 2, in which I denotes the inner wall of the flow chamber and O the outer wall of the flow chamber. Thereby, the signal generated by the cells passing through such an illuminated region will correspondingly comprise sidelobs P(1) and P(2) at both sides of the main pulse signal P. If the amplitudes of these pulses are also to be identified as the scattered signals of cells, the result of the sample detection would be incorrect.
In a prior art approach for resolving this problem, a scattered pulse signal recognition algorithm is incorporated in the signal processing unit, getting rid of the sidelobs via a threshold value Vth. As shown in FIG. 3, when a peak as recognized is smaller than this threshold value, it is deemed to be a false signal and is rejected. However, this approach has a significant disadvantage in that usually the main pulse Pk of some smaller cells is even smaller than the associated pulse of the bigger cells, and the real scattered signal Pk of the small cells would be rejected along with the associated pulse of the bigger cells by using the threshold value, which will bring error to the cell analysis.
As disclosed in U.S. Pat. No. 6,713,019, a negative cylindrical lens (concave cylindrical lens) is used for eliminating the aberration in y direction so as to avoid the disturbance of the associated pulse. However, this approach has the following drawbacks: the complexity of the optical system, the dimension of the structure, and the difficulties upon assembly and adjustment are all increased; while minimizing the error in y direction, the concave cylindrical lens also influences the x direction, as a result of which the flat top effect upon light beam shaping is poor, which further affects the overall performance of the system; for certain systems requiring irradiation with multi-wavelength, the compensation effect of such a concave cylindrical lens is inconsistent, and it is still needed to increase the complexity of the signal recognition algorithm.
As disclosed in U.S. Pat. No. 5,788,927, an aspheric lens is used for collimating the light beam from the semiconductor laser, which is then focused via a space filter and a spherical lens to create an elliptical spot that irradiates into the flow cell. Although this approach also functions to eliminate the sidelobs, the application of the space filter enlarges the size of the structure and increases the difficulty upon assembly and adjustment. Furthermore, one spherical lens is incapable of forming a flat top in x direction, leading to aberration if the cells are introduced into the irradiated region from different positions.